This invention generally relates to a method of machining a part using a turning machine having non-rotating locating centers for support of a part to be machined thereon. More particularly, the invention relates to a method and apparatus for machining a hardened part, the hardened part being mounted on non-rotating locating centers to reduce runout of the machined part.
A wide variety of mechanical parts are initially forged from a steel or like material in an unmachined configuration, and thereafter the forged part is machined to a final configuration for use. Conventionally, a part is machined prior to subjecting the part to a heat-treatment stage for hardening of the part to extend its useful life and function for which the part was made. In these cases, a conventional approach was to machine a conical center bore in the initially forged unmachined configuration, the center bore receiving a tool center for mounting of the part on locating centers of a turning machine or lathe. The locating centers are then conventionally turned at high speed for rotation of the part. Subsequent to this, in order to reduce cost of a part, the unmachined part was forged with a conical center bore for receiving the locating centers. Upon subsequent machining, it has been found that holding very exacting tolerances on a machined part is difficult due to imperfect machined or forged center bores in a part, which may result in wobbling of the part upon turning in the machining process. Various methods and constructions have been proposed to increase the accuracy of the machining process and to avoid the difficulties of imperfect conical center bores used to mount a part to be machined. Particularly, the configuration of the center bore may allow an unmachined part to adjust for distortions occurring in the center bore.
Although the configuration of the center bore forged or machined into a part can facilitate machining with high accuracy, it has been found that the turning machine or lathe used in machining of a part itself may contribute to unacceptable inaccuracies in a final machined part. Typically, a lathe or turning machine will include live centers which are turned within a bearing set. These live centers support a workpiece for rotation about an axis, and are fixed to the workpiece and driven about the axis to cause the workpiece to rotate. Such live centers contribute to excessive runout in a finely-machined part, caused by the bearing set supporting the rotating machine center. These runout errors increase upon continued use of the turning machine with continued wear of the bearings supporting the lathe center. For example, after a turning machining or lathe has been used for a period of time, the minimum amount of runout achievable has been found to be approximately 0.002 to 0.003. In complex machined parts, such as hypoid gear sets, it is desired to hold tolerances of a final machined part to 0.001 for example. Typically, the fabrication of a final part will include machining of an untreated forged part which will then be subjected to a heat treatment process for hardening of the material. Subsequent to heat treatment, an attempt to straighten the part will result in bending of further runout into a part instead of the desired straightening, particularly if initial runout in the machined part is greater than required tolerances.
It has also been found that the step of heat treating a machined part subsequent to the machining process may induce additional errors resulting in unacceptable part quality. It would therefore be desirable to perform heat treating or hardening of the part prior to machining to avoid such errors. Conventionally, after a hardening process is performed, any finish machining of a part required grinding on a grinding machine, as tooling for machining a hardened part on a turning machine or lathe was not available. Upon the development of tooling such as ceramic or cubic boron nitride cutting tools in use with a turning machine, it is now possible to turn hardened parts accordingly. Turning of hardened parts reduces cost as compared to grinding of hard parts, and therefore would be desirable if problems with induced runout as noted above can be overcome.